1. Field of the Invention
This invention relates to an auto-tracking apparatus used in rotary head magnetic recording/reproducing apparatus, and more particularly to a tracking-error correcting system for securing the accurate operation of the auto-tracking apparatus.
2. Description of the Prior Art
In a rotary head magnetic recording/reproducing apparatus (VTR, hereafter) it is necessary to detect tracking error between a reproducing head and a recorded track and to move the reproducing head in the direction of the axis of rotation so that the reproducing head is placed at a better tracking position. Such a system is called the `auto-tracking system`. The piezoelectric element is often used for supporting the magnetic heads. But, generally speaking, the strain of the piezoelectric element does not change linearly for the applied voltage, especially when the voltage is high level, the nonlinearity increases significantly. This means that tracking-error increases in proportion to the increase of the applied voltage. In addition, the sensitivity of the piezoelement degrades gradually as the time passes, so that the nonlinearity increases more significantly. Accordingly, the auto-tracking cannot be completed accurately.
The conventional auto-tracking system samples and holds the envelope of the RF signal from the VTR, and changes (e.g. increases) the control voltage to the piezoelectric element, and then compares the envelope level in the next frame with the held envelope level in the previous frame. As a result, if the next envelope level is higher than the previous envelope level, the control voltage will be further increased, and continue comparing the envelope level with further (next) frame's envelopes. In other words, the control voltage will be changed in the same direction until the envelope level becomes lower than the previous envelope level. When the envelope level is lower than the held envelope level of the previous frame, the direction of the control voltage will be inverted.
In such a conventional auto-tracking system, however, it is necessary to detect the change of the envelope by one step change of the control voltage in each frame. But when the change of the envelope according to the change of sensitivity of the piezoelectric elements exceeds the change of the envelope according to the change of the control voltage, the auto-tracking system cannot avoid to make a mistake. Accordingly, it is necessary that the change of the envelope according to one step change of the control voltage is larger than the change of the envelope according to the change of the piezoelectric element's sensitivity.
Furthermore, if the reproduced envelope is generated abnormally, for example, the envelope level is very low, the auto-tracking system also makes a mistake. Especially, when the reproduction mode is changed, for example, from a forward fast mode to a reverse fast mode, the magnetic tape is not transferred smoothly by the radical change of velocity, so that the reproduced signals applied to a triangular wave circuit are not correct. Thus, the output signal from the triangular wave generator becomes an incorrent signal. Accordingly, the reproduced envelope is improper, so that the output signal from the auto-tracking circuit becomes improper.
Furthermore, even if the output signal from the triangular wave generator recovers to a correct signal, the reproduced envelope is not generated normally at once, because the output signal from the auto-tracking circuit is not correct.